I have the exact same issue on Windows 8x64, but have an nVidia GPU.
I also reverted back to Flash version 10.3, where HD videos work perfectly.
The issue still exists on flash version 11.8.800.94 in Firefox.
For me it is only in Firefox that the issues exists, IE plays ok for me.
Anyone know when this will be fixed?
it seems that the problem might not be from adobe side after all!
Windows 8 user,
open firefox and type about:config in the address bar and hit enter. Type “webgl” into the Filter box and then double click on the webgl.force-enabled option to change its value from false to true.
Problem solve...Radeon HD-7310 dont know for Nvidia but i assume it will work to.